1. Field of the Invention
This invention relates generally to hybrid circuits, and more particularly, to methods of reducing circuit failures due to thermal contraction mismatch between the different layers of the hybrid circuit.
2. Description of the Related Art
Hybrid circuits are made up of two or more distinct circuit layers that are typically fabricated separately, but are then joined together physically and electrically to form a device. For example, a photodetector layer and a readout IC (ROIC) layer may be packaged together and electrically interconnected to form a hybrid focal plane array (FPA) device.
Such devices are often subjected to temperatures that change over time. If the interconnected layers react differently to changes in temperature—i.e., if there is, for example, a “thermal contraction mismatch” between the layers—the device may fail at the interface between the layers due to the stress on the interconnections under thermal stress. One approach to minimizing such failures is by attempting to carefully match the thermal contraction characteristics of the interconnected layers. Another technique requires using thermal matching structures to compress one layer in an attempt to match its contraction to that of another layer. These approaches may be adequate for a few hundred thermal cycles, but are unlikely to prevent failures at the interface for devices subjected to thousands of cycles.
Another approach that has been taken to reduce thermal cycling failures at the interface between layers is to introduce an adhesive such as a low-viscosity non-conductive epoxy between the layers, which serves to hold the layers together and to relieve stress on the interconnections between the layers. However, such an adhesive ‘backfill’ may also tend to degrade the performance of any circuit elements contacted by the epoxy. For example, for a hybrid FPA device, epoxy backfill that comes into contact with the individual photodetectors on the photodetector layer may degrade the detector's performance. This can be particularly problematic when the photodetector layer is non-planar—i.e., with raised “mesa” features that are separated by lower regions—as such photodetector layers are often especially susceptible to degradation due to contact with an epoxy backfill.